Conventionally, there is known a fixing device in which an induction heating coil is situated in a roller outside a heat roller. A fixing device is known which fixes an image on a sheet using a pair of heat rollers forming a nip and a pair of induction heating coils.
In order to secure the transport path of the sheet, two coils are laid out so that the coil surfaces face a roller outer peripheral surface, respectively. The fixing device should lay out two coils in a place separated from the nip.
For low electric power consumption, it is preferable that the time until the temperature of a belt reaches a fixing temperature is short. The belt starts to run. A belt portion, in which the coil is heated, moves from a rotation upstream of the heat roller to a rotation downstream thereof and approaches the nip.
However, before the warm belt portion reaches the nip, the heat radiates from the belt into the air. The fixing device continues to supply electric power to the coil.
Given the long warming-up time, wastage of electric power occurs. A further short warming-up time is required in the image forming device.